The research will pursue two fundamental issues of longstanding scientific interest: (1) Can true symbolic relations be demonstrated in individuals who are not considered to function symbolically? (2) If so, does that potential depend on development of some degree of language? This project will focus on two populations, school-aged children with severe mental retardation and little or no language and capuchin monkeys. We will test the hypothesis that both populations have the potential to function symbolically. For our purposes, symbolic potential will be defined as positive outcomes on tests for stimulus equivalence. We will also examine the possibility that probable precursors of symbolic functioning (e.g., exclusion and contingency classes) will be demonstrable also. Our hypothesis is that past failures to demonstrate these behavioral capacities in the various populations have been due to methodological insufficiency: insufficient specification of stimuli controlling the participants behavior, insufficient preparation for critical tests, and insufficient control of motivational variables. Using methods from recent research on stimulus control, we seek to: (1) demonstrate the behavioral prerequisites for stimulus equivalence, exclusion, and contingency classes in both populations; (2) test a novel account of processes involved in decontextualization, another index of symbolic potential, based on the interaction of two types of equivalence classes; and (3) develop a useful animal model for studying symbolic processes in the laboratory (e.g., to assist in the long-term search to understand the neurological foundation of these processes). Our studies are expected to have direct implications for both theory and practice. A competent demonstration of symbolic functioning in presymbolic populations would necessarily lead to modifications in theories that relate such functioning to language development. Perhaps more importantly, our methods are likely to prove useful in the design of teaching interventions for individuals who do not yet exhibit conventional symbolic functioning.